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Services and Control of The Smart Transformer 

Marco Liserre, Zhixiang Zou and Giovanni De Carne

Brief Introduction:

    The increasing connection of renewables and new loads is challenging the distribution grids. For overcoming actual and foreseen challenges, a new concept, with the capability to form intelligent grid nodes, is proposed: the “Smart Transformer”. The Smart Transformer is a power electronics-based transformer, aiming not only to adapt the voltage level from MV to LV grids but also providing ancillary services to the grid. In order to exploit its capability, the ST requires combining power system aspects and power electronics constraints, resulting in new requirements and challenges.

    The Smart Transformer provides galvanic isolation, voltage adaptation and the capability to control the reactive power in grid feeding operation for sustaining the voltage profile in grid. On the Low Voltage side, it can interact with loads and generators by means of voltage and frequency control, thereby controlling the power generation and the consumption in the grid. Additionally, the Smart Transformer is a natural connection point for hybrid (AC and DC) grids both at MV and at LV levels, being an enabler for the DC connectivity. Among the others for storage integration and for EV fast charging station integration. Nevertheless, the application of the ST requires an optimization of the system for minimum costs, maximum availability and high efficiency.

    Though the ST can well address system-level issues, challenges from the point of view of voltage control are encountered. The LV converter of the ST is essentially a grid-forming converter. Comparing to the ideal slack bus, it has limited control bandwidth and therefore can interact with the the local appliances in the LV grid, leading to instability and power quality violation. In particular, the interactions between the ST voltage controller and the key components of the local grid converters, for instance, local power/current control, filter resonances, and grid synchronisation. In addition, design of voltage control during grid service provision (e.g., frequency variations) is another key issue, which affects the performance of the grid operation.

    This tutorial introduces the Smart Transformer concept and takes into account power system considerations as well as power electronics knowledge. ST architectures and topologies, basic controller designs and innovative concepts for increasing the availability are introduced. The advanced features, which the Smart Transformer offers to the AC grids are examined. In particular, the focus is put on voltage and frequency control in grid forming operation, load sensitivity identification, load control, and reactive power support in grid feeding operation. Meanwhile, the control-level issues of ST-fed grid are addressed. For the control interaction, an optimal voltage control as well as design consideration is proposed for ST with stability concern. Based upon the optimal control, the filter-based active damping as well as virtual impedance is integrated, in order to alleviate grid resonances and synchronization issue. Moreover, frequency adaptive control is embedded with the voltage control so that the system can maintain high power quality during service provision.

    The Project will summarize the results of EU ERC Consolidator Grant Project “HEART” and update on current projects promoted by Distributed System Operators leading to testing the Smart Transformer technology in the field.  

image.pngMarco Liserre received the M.Sc. and Ph.D. degrees in electrical engineering from the Polytechnic University of Bari, Bari, Italy, in 1998 and 2002, respectively. He has been an Associate Professor with the Bari Polytechnic and, since 2012, a Professor in Reliable Power Electronics with Aalborg University, Denmark. Since 2013, he has been a Full Professor, and he holds the Chair of Power Electronics at Kiel University, Germany, where he leads a team of more than 20 researchers with an annual budget cooperation with 20 companies. He has authored of 2 Million Euro and or coauthored more than 350 technical papers (more than 110 of them in international peer-reviewed journals) and a book.

Dr. Liserre’s works have received more than 25 000 citations. He is listed in the ISI Thomson report “The world’s most influential scientific minds” from 2014. He has been awarded with an ERC Consolidator Grant for the project ”The Highly Efficient And Reliable smart Transformer (HEART), a New Heart for the Electric Distribution System” and with the ERC Proof of Concept Grant U-HEART. He is a member of Industry Application Society (IAS), Power Electronics Society (PELS), Power and Energy Society (PES), and Industrial Electronics Society (IES). He has been serving all these societies in different capacities. He has received the IES 2009 Early Career Award, the IES 2011 Anthony J. Hornfeck Service Award, the 2014 Dr. Bimal Bose Energy Systems Award, the 2011 Industrial Electronics Magazine best paper award the Third Prize paper award by the Industrial Power Converter Committee at IEEE Energy Conversion Congress and Exposition 2012, 2012; the 2017 IEEE PELS Sustainable Energy Systems Technical Achievement Award; and the 2018 IEEE IES Mittelmann Achievement Award, which is the highest award of the IEEE-IES.

image.pngZhixiang Zou received the B.Eng. and Ph.D. degrees in electrical and engineering from Southeast University, Nanjing, China, in 2007 and 2014, respectively, the Dr.-Ing. degree (summa cum laude) from Kiel University, Germany. He was an engineer in the State Grid Electric Power Research Institute, Nanjing, China, from 2007 to 2009. He was a research fellow at the Chair of Power Electronics, Kiel University, Germany, from 2014 to 2019. Since October 2019, he has been with the School of Electrical Engineering, Southeast University. His research interests include smart transformers, microgrid stability, and control of grid converters.

Dr. Zou serves as an Associate Editor of the IEEE Open Journal of Power Electronics, an Associate Editor of the IEEE Access, an Editor of the International Transactions on Electrical Energy Systems, and an Editor of the Mathematical Problem in Engineering.

image.pngGiovanni De Carne received the M.Sc. in Electrical Engineering from Polytechnic University of Bari, Bari (Italy) in 2013. Since 2013, he carried out his Ph.D. studies at the Chair of Power Electronics at Kiel University, Germany. He defended his doctoral thesis in 2018, and continued working at Kiel university as post-doc on HVDC control and services until 2019. He is currently a Young Scientist at the Institute for Technical Physics at the Karlsruhe Institute of Technology, working on large-scale power hardware in loop systems. He is associate editor of Springer Journal “Electrical Engineering”. He is member of PELS, IES and PES, where is an active author and reviewer of journals and conferences of these societies.